The present invention relates generally to process control networks and, more specifically, to a method of and to an apparatus for implementing redundant devices in a process control network.
Large processes such as chemical, petroleum, and other manufacturing and refining processes include numerous field devices disposed at various locations to measure and control parameters of the process to thereby effect control of the process. These field devices maybe, for example, sensors such as temperature, pressure, and flow rate sensors as well as control elements such as valves and switches.
Historically, the process control industry used manual operations like manually reading level and pressure gauges, turning valve wheels, etc., to operate the measurement and control field devices within a process. Beginning in the 20th century, the process control industry began using local pneumatic control, in which local pneumatic controllers, transmitters, and valve positioners were placed at various locations within a process plant to effect control of certain plant locations. With the emergence of the microprocessor-based distributed control system (DCS) in the 1970""s, distributed electronic process control became prevalent in the process control industry.
As is known, a DCS includes an analog or a digital computer, such as a programmable logic controller, connected to numerous electronic monitoring and control devices, such as electronic sensors, transmitters, current-to-pressure transducers, valve positioners, etc. located throughout a process. The DCS computer stores and implements a centralized and, frequently, complex control scheme to effect measurement and control of devices within the process to thereby control process parameters according to some overall control scheme. Usually, however, the control scheme implemented by a DCS is proprietary to the DCS controller manufacturer which, in turn, makes the DCS difficult and expensive to expand, upgrade, reprogram, and service because the DCS provider must become involved in an integral way to perform any of these activities. Furthermore, the equipment that can be used by or connected within any particular DCS maybe limited due to the proprietary nature of DCS controller and the fact that a DCS controller provider may not support certain devices or functions of devices manufactured by other vendors.
To overcome some of the problems inherent in the use of proprietary DCSs, the process control industry has developed a number of standard, open communication protocols including, for example, the HART(copyright), PROFIBUS(copyright), WORLDFIP(copyright), LONWORKS(copyright), Device-Net(copyright), and CAN protocols, which enable field devices made by different manufacturers to be used together within the same process control network. In fact, any field device that conforms to one of these protocols can be used within a process to communicate with and to be controlled by a DCS controller or other controller that supports the protocol, even if that field device is made by a different manufacturer than the manufacturer of the DCS controller.
Moreover, there is now a move within the process control industry to decentralize process control and, thereby, simplify DCS controllers or eliminate the need for DCS controllers to a large extent. Decentralized control is obtained by having process control devices, such as valve positioners, transmitters, etc. perform one or more process control functions and by then communicating data across a bus structure for use by other process control devices in performing other control functions. To implement these control functions, each process control device includes a microprocessor capable of performing one or more control functions as well as communicating with other process control devices using a standard and open communication protocol. In this manner, field devices made by different manufacturers can be interconnected within a process control network to communicate with one another and to perform one or more process control functions forming a control loop without the intervention of a DCS controller. The all digital, two-wire bus protocol now being promulgated by the Fieldbus Foundation, known as the FOUNDATION(trademark) Fieldbus (hereinafter xe2x80x9cFieldbusxe2x80x9d) protocol is one open communication protocol that allows devices made by different manufacturers to interoperate and communicate with one another via a standard bus to effect decentralized control within a process.
The bus includes different sections, or segments, which are separated by bridge devices, such as controllers. Each segment interconnects a subset of the devices attached to the bus to enable communications between the devices to control the process. The controllers communicate with the devices on the segments via input/output (I/O) devices. The I/O devices implement the communications protocol used in the process control network, and control the communications between the controllers and the devices on the segments. Although the I/O devices facilitate the communications between the controllers and the devices on the segments, process control ceases, at least with respect to the devices on a particular segment, if the I/O device for the segment goes out of service for whatever reason.
The impact of a disabled I/O device and disruption to process control may be reduced by providing a backup I/O device that is connected to the segment and that takes over for the disabled I/O device. However, the transition from the disabled I/O device to the backup I/O device is not seamless, and disruption in the process control still occurs. Currently known backup I/O devices are not updated with the current information stored in the primary I/O device, such as current values of process variables, functional software that may reside in the I/O device, the communication schedules for the devices on the segment, and the like. Also, in some implementations, the backup I/O device does not automatically assume control when the primary I/O device becomes disabled, resulting in a delay in performing process control until a user activates the backup I/O device. Moreover, in some communications protocols such as the Fieldbus protocol, the devices are configured to communicate specifically with the primary I/O device and must be reconfigured to communicate with the backup I/O device before the backup I/O device can take over communications on the segment. Therefore, a need exists for a redundant bus I/O device in process control networks that seamlessly and transparently takes over for the disabled primary I/O device without disrupting the implementation of process control in the process control network.
The present invention is directed to a method of and a device for implementing redundant devices in a process control network such as one having distributed control functions. The method and device of the present invention use a pair of redundant devices, such as redundant I/O devices, communicatively connected to a segment of the bus in parallel to each other and in series between the controller and field devices on the segment. The redundant devices are assigned a virtual publishing address, which may be the unique address of one of the redundant devices, that is used in communicating over the bus. At all times, one of the redundant devices is operating in an active mode and communicating with the devices in process control network, and the other redundant device is operating in a backup mode wherein the redundant device maintains a communication connection with the devices and listens for messages transmitted by the devices and intended for the redundant devices, but does not respond to messages from the other devices until the device switches to the active mode. Both redundant devices transmit messages over the bus using the virtual publishing address so that their messages are identical and will be received and processed by the field devices regardless of which redundant device is active. The redundant devices are configured to detect when the active device is, or is about to become, disabled, and the backup device automatically converts to the active mode and communicates with the controller and field devices.
By using the virtual publishing address in their communications, the redundant devices are viewed by the other devices as a single virtual device that communicates continuously even when one of the physical devices is taken out of service for whatever reason. Because both redundant devices transmit messages using the virtual publishing address, the field devices do not have to be reprogrammed to receive messages with the backup redundant device""s address when the backup becomes the active redundant device, thereby reducing or eliminating the disruption to process control and communications when the active redundant device goes out of service.
According to one aspect of the present invention, each device, including the redundant devices, has a publishing buffer for storing massages to be transmitted on the bus, and a subscribing buffer for storing messages received from other devices. The redundant device operating in the backup mode receives in its subscribing buffer messages that published by the other devices and intended for the virtual device. Moreover, the backup redundant device receives messages transmitted by the active redundant device over the bus and stores these messages in its publishing buffer. By processing and storing the messages in this manner, the backup redundant device has the current information necessary to take over as the active redundant device immediately without user or system intervention to reprogram the backup redundant device with the current information.
In another aspect of the present invention, the redundant devices detect that they are part of a redundant pair based on their physical connections to the bus and to each other. In one alternative embodiment, the redundant devices are communicatively linked to each other via a direct communications link external to the bus. The redundant devices may exchange status information, process data, alarm messages, and the like, over the communications link. Alternatively, the redundant devices may be configured to detect the connection port or slot, or the configuration of the connection port or slot, to which the redundant devices are attached and thereby determine that they are one of a pair of redundant devices and that they are either the active or the backup redundant device. Configured in this way, disabled redundant devices can be replaced, with the replacement redundant device becoming operational as part of the virtual device without the necessity of reconfiguring the redundant device to operate as part of the redundant pair of redundant devices.
In a still further aspect of the present invention, the other devices in the system, such as a host device having a user interface and graphical display, may be configured to detect the configuration of the connection ports or slots for the redundant devices and the devices"" connections thereto. Once detected, the host device can format the graphical display with information indicating the presence of the redundant devices, the operating mode of each of the redundant devices, the operational status of the redundant devices, and the like.
The features and advantages of the invention will be apparent to those of ordinary skill in the art in view of the detailed description of the preferred embodiment, which is made with reference to the drawings, a brief description of which is provided below.